linux-ext4 - Re: [PATCH v6 07/19] mm: Put readahead pages in cache earlier

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Message-ID: <20200219005915.GV10776@dread.disaster.area>
Date:   Wed, 19 Feb 2020 11:59:15 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        ocfs2-devel@....oracle.com, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v6 07/19] mm: Put readahead pages in cache earlier

On Tue, Feb 18, 2020 at 07:42:22AM -0800, Matthew Wilcox wrote:
> On Tue, Feb 18, 2020 at 05:14:59PM +1100, Dave Chinner wrote:
> > On Mon, Feb 17, 2020 at 10:45:52AM -0800, Matthew Wilcox wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> > > 
> > > At allocation time, put the pages in the cache unless we're using
> > > ->readpages.  Add the readahead_for_each() iterator for the benefit of
> > > the ->readpage fallback.  This iterator supports huge pages, even though
> > > none of the filesystems to be converted do yet.
> > 
> > This could be better written - took me some time to get my head
> > around it and the code.
> > 
> > "When populating the page cache for readahead, mappings that don't
> > use ->readpages need to have their pages added to the page cache
> > before ->readpage is called. Do this insertion earlier so that the
> > pages can be looked up immediately prior to ->readpage calls rather
> > than passing them on a linked list. This early insert functionality
> > is also required by the upcoming ->readahead method that will
> > replace ->readpages.
> > 
> > Optimise and simplify the readpage loop by adding a
> > readahead_for_each() iterator to provide the pages we need to read.
> > This iterator also supports huge pages, even though none of the
> > filesystems have been converted to use them yet."
> 
> Thanks, I'll use that.
> 
> > > +static inline struct page *readahead_page(struct readahead_control *rac)
> > > +{
> > > +	struct page *page;
> > > +
> > > +	if (!rac->_nr_pages)
> > > +		return NULL;
> > 
> > Hmmmm.
> > 
> > > +
> > > +	page = xa_load(&rac->mapping->i_pages, rac->_start);
> > > +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > > +	rac->_batch_count = hpage_nr_pages(page);
> > 
> > So we could have rac->_nr_pages = 2, and then we get an order 2
> > large page returned, and so rac->_batch_count = 4.
> 
> Well, no, we couldn't.  rac->_nr_pages is incremented by 4 when we add
> an order-2 page to the readahead.

I don't see any code that does that. :)

i.e. we aren't actually putting high order pages into the page
cache here - page_alloc() allocates order-0 pages) - so there's
nothing in the patch that tells me how rac->_nr_pages behaves
when allocating large pages...

IOWs, we have an undocumented assumption in the implementation...

> I can put a
> 	BUG_ON(rac->_batch_count > rac->_nr_pages)
> in here to be sure to catch any logic error like that.

Definitely necessary given that we don't insert large pages for
readahead yet. A comment explaining the assumptions that the
code makes for large pages is probably in order, too.

> > > -		page->index = offset;
> > > -		list_add(&page->lru, &page_pool);
> > > +		if (use_list) {
> > > +			page->index = offset;
> > > +			list_add(&page->lru, &page_pool);
> > > +		} else if (add_to_page_cache_lru(page, mapping, offset,
> > > +					gfp_mask) < 0) {
> > > +			put_page(page);
> > > +			goto read;
> > > +		}
> > 
> > Ok, so that's why you put read code at the end of the loop. To turn
> > the code into spaghetti :/
> > 
> > How much does this simplify down when we get rid of ->readpages and
> > can restructure the loop? This really seems like you're trying to
> > flatten two nested loops into one by the use of goto....
> 
> I see it as having two failure cases in this loop.  One for "page is
> already present" (which already existed) and one for "allocated a page,
> but failed to add it to the page cache" (which used to be done later).
> I didn't want to duplicate the "call read_pages()" code.  So I reshuffled
> the code rather than add a nested loop.  I don't think the nested loop
> is easier to read (we'll be at 5 levels of indentation for some statements).
> Could do it this way ...

Can we move the update of @rac inside read_pages()? The next
start offset^Windex we start at is rac._start + rac._nr_pages, right?

so read_pages() could do:

{
	if (readahead_count(rac)) {
		/* do readahead */
	}

	/* advance the readahead cursor */
	rac->_start += rac->_nr_pages;
	rac._nr_pages = 0;
}

and then we only need to call read_pages() in these cases and so
the requirement for avoiding duplicating code is avoided...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com